Engine oil, or engine lubricant is any various substances comprising base oils enhanced with particularly antiwear additive plus detergents, dispersants and, for multi-grade oils viscosity index improver. Motor oil is used for lubrication of internal combustion engines.

The main function of motor oil is to reduce friction and wear on moving parts and to clean the engine from sludge (one of the functions of dispersants) and varnish (detergents). It also neutralizes acids that originate from fuel and from oxidation of the lubricant (detergents), improves sealing of piston rings, and cools the engine by carrying heat away from moving parts.

In addition to the basic constituents noted in the preceding paragraph, almost all lubricating oils contain corrosion (GB: rust) and oxidation inhibitors. Motor oil may be composed of only a lubricant base stock in the case of non-detergent oil, or a lubricant base stock plus additives to improve the oil’s tendency, extreme pressure performance, and ability to inhibit corrosion of engine parts.

Motor oils today are blended using base oils composed of petroleum-based hydrocarbons, polyalphaolefins (PAO) or their mixtures in various proportions, sometimes with up to 20% by weight of esters for better dissolution of additives.

Motor oil is a lubricant used in internal combustion engines, which power cars, motorcycles, lawnmowers, engine-generators, and many other machines. In engines, there are parts which move against each other, and the friction wastes otherwise useful power by converting the kinetic energy to heat. It also wears away those parts, which could lead to lower efficiency and degradation of the engine. This increases fuel consumption and decreases power output and can lead to engine failure.